Optical semiconductor device having waveguide layers buried in an InP current blocking layer

ABSTRACT

An integrated semiconductor laser produced by forming waveguide layers each having a particular band gap and a particular layer thickness collectively and then forming an InP current blocking layer. After an InGaAsP layer has been formed on an InP substrate, a waveguide including a multiple quantum well active layer is formed by selective MOVPE. Then, the waveguide is buried in an InP current blocking layer. In this configuration, the current blocking layer exhibits its expected function without regard to the width of SiO 2  stripes used for selective metalorganic vapor phase epitaxial growth (MOVPE). The laser is feasible for high output operation and can be produced at a high yield.

BACKGROUND OF THE INVENTION

The present invention relates to an optical semiconductor device and amethod of producing the same and, more particularly, to an integratedsemiconductor device feasible for high temperature or high opticaloutput operation because of a minimum amount of current leak to occurfrom its current blocking layer, and a method of producing the same.

Today, the applicable range of optical communication is extending fromtrunk lines to branch lines or even to access lines. Semiconductorlasers for optical communication are therefore required to have advancedperformance and new functions. For a trunk line, for example, asemiconductor laser whose drive spectrum involves a minimum of jitter atthe time of modulation is essential in order to implement high speed,long distance transmission. A distributed feedback (DFB) laser having anelectro absorption type optical modulator integrated therewith is onetype of semiconductor laser meeting the above requirement. As for anaccess line, there is an increasing demand for a semiconductor laser tobe easy to mount and capable of being efficiently coupled to an opticalfiber. This kind of laser may be typified by a so-called spot sizeconversion type laser including a portion for converting the size of abeam.

Japanese Patent Laid-Open Publication No. 62-102583, for example,teaches a semiconductor laser desirable in high temperature or highoptical output characteristic. However, the problem with the lasertaught in this document is that the characteristic and yield are limitedfor the following reasons. To form layers different in band gap andthickness on a single substrate, crystal growth and crystal etching mustbe repeated, increasing the number of steps. Moreover, it is difficultto connect waveguides different in band gap and thickness accurately. Asa result, a light loss occurs at the connecting portion and deterioratesthe optical output characteristic.

To implement an integrated semiconductor laser, selective metalorganicvapor phase epitaxial growth (MOVPE) procedure capable of formingwaveguide layers different in band gap and thickness collectively isattracting increasing attention. A semiconductor laser produce byselective MOVPE is disclosed in, e.g., the Papers of 56th JapaneseScience Lecture Meeting, 1995, 27p-ZA-7, p. 930. This kind of scheme,however, brings about a problem that a current blocking layer includedin the laser cannot function sufficiently in a high temperatureenvironment or on the injection of a great current. Specifically, whilea laser portion usually has the smallest band gap, it is necessary withselective MOVPE to increase the width of an anti-growth mask in thewaveguide portion where the band gap should be reduced, i.e., the laserportion. It follows that in the laser portion the current blocking layerimplemented only by InP and having a p-n-p-n thyristor structure has abroad area at both sides of an active layer. As a result, currentleakage from the current blocking layer and ascribable to the storage ofelectrons is aggravated.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide anintegrated semiconductor laser feasible for high temperature or highoutput operation, and a method of producing the same.

In accordance with the present invention, in an optical semiconductordevice having an electro absorption type modulator and a DFB laser in anintegrated structure, the modulator and laser are formed on an InPsubstrate formed with an InGaAsP layer, and waveguide layersrespectively functioning as the modulator and laser are buried in an InPcurrent blocking layer.

Also, in accordance with the present invention, in an opticalsemiconductor device including a laser having a spot size convertingportion integrally therewith, the laser is formed on an InP substrateformed with an InGaAsP layer, and waveguide layers respectivelyfunctioning as the laser and spot size converting portion are buried inan InP current blocking layer.

Further, in accordance with the present invention, a method of producinga semiconductor device has the steps .of forming two SiO₂ stripes on anInP substrate formed with a n InGasP layer, and forming a waveguideincluding a multiple quantum well active layer by selective MOVPEbetween the two stripes, and burying the waveguide in an InP currentblocking layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become apparent from the following detailed descriptiontaken with the accompanying drawings in which:

FIG. 1 shows a conventional semiconductor laser having an improved hightemperature or high optical output characteristic;

FIG. 2 shows another conventional semiconductor laser implemented byselective MOVPE;

FIGS. 3A and 3B show a semiconductor laser embodying the presentinvention and implemented as a DFB laser including a semiconductormodulator integrally therewith; and

FIGS. 4A and 4B show an alternative embodiment of the present inventionand implemented as an integrated semiconductor laser including a spotsize converting portion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To better understand the present invention, brief reference will be madeto a conventional semiconductor laser featuring an improved hightemperature or high optical output characteristic, shown in FIG. 1. Thelaser to be described is taught in Japanese Patent Laid-Open PublicationNo. 62-102583 mentioned earlier. As shown, the laser includes a(100)n-InP substrate 301. An n-InGaAsP carrier recombination layer 302,an n-InP cladding layer 303, an InGaAsP active layer 304 and a p-InPcladding layer 305 are sequentially formed on the substrate 301 byliquid phase epitaxial growth (LPE hereinafter). Two parallel groovesreaching the recombination layer 302 are formed in a <011> direction,implementing a mesa stripe. Subsequently, a p-InP/n-InP/p-InP currentblocking layer 306 having a thyristor structure and a p-InGaAsP caplayer 307 are formed by LPE.

The current blocking layer 306 includes a carrier recombination layerimplemented by InGaAsP having a smaller band gap than InP. With theblocking layer 306, the semiconductor laser restricts the storage ofelectrons in the n-InP blocking layer during high temperature operationor high optical output operation. This successfully obviates the turn-onof the current blocking layer 306 having the p-n-p-n thyristorstructure. However, the configuration shown in FIG. 1 results in limitedyield and limited optical output characteristic for the reasonsdiscussed earlier.

FIG. 2 shows another conventional semiconductor laser which is producedby selective MOVPE. As shown, the laser includes a (100)n-InP substrate401. A pair of SiO₂ (silicon dioxide) stripe masks are formed on thesubstrate 401 at an interval of 5 μm in a <011> direction. Then, ann-InP buffer layer 402, an active layer 403 formed of InGaAsP and havinga multiple quantum well (MQW layer hereinafter) structure, and an activestripe implemented by a p-InP cladding layer 404 are formed by selectiveMOVPE. Subsequently, a current blocking layer 405 implemented as ap-InP/n-InP/p-InP layer and having a thyristor structure and a p+-InGaAscap layer 406 are formed. This kind of laser configuration is disclosedin the Papers of 56th Japanese Science Lecture Meeting, 1995, 27p-ZA-7,p. 930.

The laser shown in FIG. 2 allows its SiO₂ stripe mask width to bemodulated in order to change the band gap and/or thickness of the MQWlayer. It is therefore possible to form waveguides each having aparticular function by a single crystal growth. However, the problem isthat the current blocking layer 405 cannot exhibit its expected functionin a high temperature environment or on the injection of a large amountof current.

Referring to FIGS. 3A and 3B, a semiconductor device embodying thepresent invention is shown and implemented as a DFB laser having anintegrated field electro type modulator integrally therewith by way ofexample. As shown, the DFB laser includes a (100) orientation n-InPsubstrate 101 having a laser portion 102 and a modulator portion 105. Aprocedure for producing the DFB laser will be described first. The laserportion 102 of the substrate 101 is formed with a corrugation 103 in a<011> direction by holographic exposure and wet etching. The corrugation103 has a period of 241.7 nm. An InGaAsP layer 104 is formed on thelaser portion 104, including the corrugation 103, by MOVPE at a pressureof 75 Torr and a temperature of 625° C. The InGaAsP layer 104 has athickness of 0.1 μm, a carrier concentration of 5×10¹ ⁷ cm⁻³, and a bandgap wavelength of 1.13 μm. Anti-growth masks 106 are formed on theInGaAsP layer 104. Specifically, after SiO₂ has been deposited on theInGaAsP layer 104 in a 150 nm thick layer by thermal CVD, the SiO₂ layeris patterned by conventional photolithography and wet etching in orderto form the anti-growth masks 106 in the <011> direction. The masks 106are spaced by 1.5 μm, and each is 18 μm wide and 500 μm long in thelaser portion 102 and 5 μm wide and 200 μm long in the modulator portion105.

Subsequently, an eight-period MQW active layer, an InGaAsP lightconfining layer and a p-InP cladding layer are sequentially formed byselective MOVPE at a pressure of 75 Torr and a temperature of 625° C. inorder to form a waveguide mesa 107. The MQW active layer is made up of a6 nm thick 0.5% compressively strained InGaAsP well layer and an 8 nmthick barrier layer having a band gap wavelength of 1.13 μm. The InGaAsPlight confining layer is 60 nm thick and has a carrier content of 5×10¹⁷cm⁻³ and a band gap wavelength of 1.13 μm. The p-Inp cladding layer is0.1 μm thick and has a carrier concentration of 5×10¹⁷ cm⁻³. As aresult, the waveguide mesa 107 is formed such that it has a band gapwavelength composition of 1.56 μm in the laser portion 102.

At this instant, the MQW layer in the modulator portion 105 has a bandgap wavelength of 1.47 μm. After the anti-growth masks 106 have beenremoved, SiO₂ is deposited on the entire wafer to a thickness of 300 nmin the form of mesas. Then, the SiO₂ layer is processed byphotolithography and wet etching to turn out an SiO₂ pattern.

Thereafter, a 0.3 μm thick p-InP layer having a carrier concentration of5×10¹⁷ cm⁻³, a 1 μm thick n-InP layer having a carrier concentration of1×10¹⁸ cm⁻³, and a 0.2 μm thick p-InP layer having a carrierconcentration of 5×10¹⁷ cm⁻³ are sequentially formed by selective MOVPEat a pressure of 75 Torr and a temperature of 625° C. After the SiO₂mask has been removed, a 1.5 μm thick p-InP layer having a carrierconcentration of 1×10¹⁸ cm³ is formed by MOVPE at a pressure of 75 Torrand a temperature of 625° C. in order to form a current blocking layer108 having a thyristor structure. A 0.2 μm thick InGaAs cap layer 109having a carrer concentration of 5×10¹ ⁸ cm⁻³ is formed on the currentblocking layer 108.

After 10 μm wide mesa stripes have been formed by wet etching, SiO₂ 110is deposited to a thickness of 350 nm and then subjected to conventionalphotolithography and wet etching in order to form contact holes. Then,Ti and Au are respectively deposited to a thickness of 100 nm and athickness of 300 nm by sputtering and then subjected to conventionalphotolithography and wet etching so as to form p-electrodes or pads 111in the modulator portion 105 and laser portion 102. After the wafer withsuch a configuration has been ground to a thickness of 100 μm, Ti and Auare respectively deposited on the rear of the wafer to a thickness of100 nm and a thickness of 300 nm by sputtering in order to form ann-electrode 112. The wafer undergone the above procedure is sintered inan N₂ atmosphere.

Finally, the wafer is cleaved at the center between the laser portion102 and the modulator portion 105. A reflection film 113 whosereflectance is as high as 90% and a reflection film 114 whosereflectance is as low as 0.1% are respectively formed on the end face ofthe laser side and the end face of the modulator side.

The laser produced by the above procedure was found to implement anoptical output of 50 mW, which is double the a conventional opticaloutput, at drive thresholds of 8 mA and 200 mA. Further, when a reversebias voltage of 2 V is applied to the modulator, an extinction ratio ofhigher than 15 dB was attained. Moreover, a penalty of less than 1 dB wa s achieved as a result of a transmission test using a modulation rateof 2.5 Gb/s and a 150 km long normal fiber.

An alternative embodiment of the present invention will be describedwith reference to FIGS. 4A and 4B. This embodiment is implemented as asemiconductor laser with a spot size converting portion. As shown, thelaser includes a (100) orientation n-InP substrate 201. How the laser isproduced will be described first. A 60 nm thick InGaAsP layer 202 whosecarrier content is 5×10¹⁷ cm-3 and band gap wavelength is 1.05 μm isformed on the substrate 201 by MOVPE at a pressure of 75 Torr and atemperature of 625° C. After SiO₂ has been deposited to a thickness of150 nm by thermal CVD, it is subjected to conventional photolithographyand wet etching in the <011> direction in order to form anti-growthmasks 203. The anti-growth masks 203 are spaced by 1.5 μm, and each is60 μm wide and 300 μm long in a laser portion 204 and 50 μm to 5 μm wideand 200 μm long in a spot size converting portion 206; the width in theportion 206 gradually varies in the above range.

Subsequently, a seven-period MQW active layer, an InGaAsP lightconfining layer and a p-InP cladding layer are sequentially formed byselective MOVPE at a pressure of 75 Torr and a temperature of 625° C inorder to form a waveguide mesa 205. The MQW active layer is made up of a4 nm thick 1% compressively strained InGaAsP well layer and an 8 nmthick barrier layer having a band gap wavelength of 1.13 μm. The InGaAsPlight confining layer is 60 nm thick and has a carrier concentration of5×10¹⁷ cm⁻³ and a band gap wavelength of 1.13 μm. The p-InP claddinglayer is 0.1 μm thick and has a carrier concentration of 5×10¹⁷ cm⁻³. Asa result, the waveguide mesa 205 is formed such that it has a band gapwavelength composition of 1.3 μm in the laser portion 204. At thisinstant, each layer is gently reduced in thickness to about one-third.

Sio₂ is deposited on the entire wafer to a thickness of 300 nm in theform of mesas. Then, the SiO₂ layer is processed by photolithography andwet etching to turn out an SiO₂ pattern. Thereafter, a 0.3 μm thickp-InP layer having a carrier concentration of 5×10¹⁷ cm⁻³, a 1 μm thickn-InP layer having a carrier concentration of 1×10¹ ⁸ cm⁻³, and a 0.2 μmthick p-InP layer having a carrier concentration of 5×10¹⁷ cm⁻³ aresequentially formed by selective MOVPE at a pressure of 75 Torr and atemperature of 625° C. After the SiO₂ mask has been removed, a 4 μmthick p-InP layer having a carrier content of 1×10¹ ⁸ cm⁻³ is formed byMOVPE at a pressure of 75 Torr and a temperature of 625° C. in order toform a current blocking layer 207 having a thyristor structure. A 0.2 μmthick InGaAs cap layer 208 having a carrier content of 5×10¹ ⁸ cm⁻³ isformed on the current blocking layer 207.

Thereafter, SiO₂ is deposited to a thickness of 350 nm and thensubjected to conventional photolithography and wet etching in order toform contact holes. Then, Ti and Au are respectively deposited to athickness of 100 nm and a thickness of 300 nm by sputtering and thensubjected to conventional photolithography and wet etching so as to formp-electrodes or pads 210. After the wafer with such a configuration hasbeen ground to a thickness of 100 μm, Ti and Au are respectivelydeposited on the rear of the wafer to a thickness of 100 nm and athickness of 300 nm by sputtering in order to form an n-electrode 211.The wafer undergone the above procedure is sintered in an N₂ atmosphere.

Finally, the wafer is cleaved at the center between the laser portion204 and the spot size converting portion 206. A reflection film 212whose reflectance is as high as 95% and a reflection film 213 whosereflectance is as low as 30% are respectively formed on the end face ofthe laser side and the end face of the spot size conversion side.

For a drive threshold of 6 mA and an optical output of 20 mW at roomtemperature, the illustrative embodiment was successful to drive thelaser with a drive current of 40 mA. Particularly, at a temperature of 85° C and for an oscillation threshold of 15 mA and an optical output of20 mW, the illustrative embodiment reduced the drive current to 75 mAwhich was more than 30% lower than the conventional drive current.Further, the laser emitted light with a radiation angle of 13°×13°,i.e., it achieved a desirable spot size conversion characteristic.

In summary, in accordance with the present invention, waveguide layerseach having a particular layer thickness and a particular compositioncan be collectively formed by selective MOVPE. This allows integratedsemiconductor lasers feasible for high temperature operation or highoutput operation to be produced at a high yield. Specifically, althoughan SiO₂ stripe mask width is modulated at the time of MOVPE in order toform the above waveguide layers, a current blocking layer capable ofexhibiting a sufficient current blocking function without regard to theabove width can be formed.

Various modifications will become possible for those skilled in the aftafter receiving the teachings of the present disclosure withoutdeparting from the scope thereof.

What is claimed is:
 1. An optical semiconductor device comprising: ann-InP substrate having a laser portion and a modulator portion, thelaser portion being formed with a corrugation; an InGaAsP layer formedon a top surface of the n-InP substrate, wherein the lnGaAsP layer isformed in a region including the corrugation; a waveguide mesa formed onboth the laser portion and the modulator portion. the waveguide mesabeing thicker in the laser portion than in the waveguide portion; and anInP current blocking laver formed above and around the waveguide mesa,wherein the waveguide mesa comprises: an eight-period MQW active layer;an InGaAsP light confning layer; and a p-InP cladding layer, wherein theInP current blocldng layer comprises: a first p-InP layer having a firstcarrier concentration; an n-InP layer having a second carrierconcentration greater than the first carrier concentration, the n-InPlayer being formed directly on the first p-InP layer; a second p-InPlayer having the first carrier concentration, the second p-InP layerbeing formed directly on the n-InP layer; and a third pInP layer havingthe second carrier concentration, the third p-InP layer being formeddirectly on the second p-InP layer.
 2. An optical semiconductor deviceas claimed in claim 1, further comprising: an InGaAs cap layer having athird carrier concentration greater than the second carrierconcentration, the InGaAs cap layer being formed directly on the InPcurrent blocking layer.
 3. An optical semiconductor device as claimed inclaim 2, further comprising: a first reflection film having areflectivity percentage greater than a predetermined amount, the firstreflection film being formed on an end face of the laser portion; and asecond reflection film having a reflectivity percentage less than thepredetermined amount, the second reflection film being formed on an endface of the modulator portion.
 4. An optical semiconductor device asclaimed in claim 3, wherein the reflectivity percentage of the firstreflection film is approximately 90%, and wherein the reflectivitypercentage of the second reflection film is approximately 0.1%.
 5. Anoptical semiconductor device as claimed in claim 3, furter comprising atleast one pad formed on the cap layer.
 6. An optical semiconductordevice as claimed in claim 3, further comprising an n-electrode formedagainst a bottom surface of the n-InP substrate.
 7. In an opticalsemiconductor device having a laser and a spot size converting portionintegrally therewith, said laser is formed on an InP substrate andformed with an InGaAsP layer. and waveguide layers respectivelyfunctioning as said laser and said spot size converting portion areburied in an InP current blocking layer that is formed above and aroundthe waveguide layers, wherein said InP current blocking laver is formedhaving a thyristor structure, wherein the waveguide layers comprise: aseven-period MOW active layer; an InGaAsP light confining layer; and ap-InP cladding layer, wherein the InP current blocking layer comprises:a first p-InP layer having a first carrier concentration; an n-InP layerhaving a second carrier concentration greater than the first carrierconcentration, the n-InP layer being formed directly on the first p-InPlayer; a second p-InP layer having the first carrier concentration, thesecond p-InP layer being formed directly on the n-InP layer; and a thirdp-InP layer having the second carrier concentration, the third p-InPlayer being formed directly on the second p-InP layer.
 8. An opticalsemiconductor device as claimed in claim 7, further comprising: anInGaAs cap layer having a third carrier concentration greater than thesecond carrier concentration, the InGaAs cap layer being formed directlyon the InP current blocking layer.